In order to generate DC voltages from AC voltages, rectifier bridges made up of an interconnection of diodes are mostly utilized at present. Alternatively, synchronous rectifiers or active rectifiers may be utilized instead of diodes, in order to reduce the conducting state power losses. In this case, suitable power MOSFETs replace the diodes which are usually utilized. This results in greater circuit complexity, since precise control with respect to time is necessary for the MOSFETs. Such additional circuitry parts are mostly combined in integrated circuits and are connected to the particular gate terminals of the MOSFETs.
A simple variant of a synchronous rectifier is described in German Patent Application No. DE 10 2007 060 219 A1. Therein, the additional circuit elements are individually assigned to each power MOSFET. The additional circuit elements are each supplied with current by a small backup capacitor in this case. If the additional circuit elements, which are composed of transistors, diodes, resistors, and the backup capacitor, are integrated together with the power MOSFET into a small housing, the component formed in this way is interconnectable like a diode.
Circuit arrangements of this type may also be designed in such a way that they allow for a limitation of high voltage peaks. High voltage peaks occur, for example, when large, sudden load changes occur in an AC generator. Such a generator may be, for example, a 3-phase or multiphase motor vehicle generator. Such a brief operating condition is referred to as a load dump.
FIG. 1 from German Patent Application No. DE 10 2007 060 219 A1 shows a rectifier circuit which is utilized instead of a silicon PN diode in a rectifier bridge. It includes not only a silicon PN diode but also a cathode terminal K1 and an anode terminal A1. MOS transistor T1 and inverse diode D6 are connected in parallel and, together in this interconnection, technically form a microelectronic component.
The rectifier circuit includes a symmetrically configured differential amplifier which is formed by transistors T2 and T3 and resistors R1, R2 and R3. A first input of this differential amplifier is connected via a diode D1 to cathode terminal K1 and to drain terminal of MOS transistor T1. A second input of this differential amplifier is connected via a diode D2 to anode terminal A1. This differential amplifier amplifies the potential difference between cathode terminal K1 and anode terminal A1 of the rectifier circuit. Due to the symmetrical configuration of the differential amplifier, temperature differences and aging effects have only a slight effect on the properties of the differential amplifier.
The output signal from the differential amplifier is available at the collector of transistor T3 and is relayed via a resistor R4 to the input of a current amplifier stage. This current amplifier stage is made up of transistors T4 and T5, the bases of which are interconnected. Zener diode D5 acts as a protective element for transistor T1 and protects its gate against overvoltages.
When an AC voltage is rectified, an AC voltage having frequency f is present between cathode terminal K1 and anode terminal A1. When there is positive potential at cathode terminal K1, MOS transistor T1, with its integrated inverse diode D6, is in the blocking mode and capacitor C1 may charge itself via diode D3 and resistor R5. The voltage present at capacitor C1 is used for supplying the further components of the rectifier circuit.
However, if the electric potential at cathode terminal K1 is more negative than the electric potential at anode terminal A1 of the rectifier circuit, the gate-source voltage of MOS transistor T1 is positive and higher than its threshold voltage. MOS transistor T1 is conductive under these conditions, a current flow having this current direction causing only a slight voltage drop.
If the electric potential at cathode terminal K1 of the rectifier circuit is again more positive than the electric potential at anode terminal A1 of the rectifier circuit, the gate-source voltage of MOS transistor T1 is less than its threshold voltage. MOS transistor T1 blocks under these conditions. For this reason, the current flow through MOS transistor T1 is only very slight.
If the electric potential at cathode terminal K1 of the rectifier circuit is more positive than the electric potential at anode terminal A1 of the rectifier circuit and this potential difference exceeds a value set by Zener diode D4, the input potential of the current amplifier stage, which is made up of transistors T4 and T5, is raised. As a result, the gate-source voltage of MOS transistor T1 also increases, and current flows between the drain and the source of MOS transistor T1. Under the given conditions, this current flow limits the electric potential difference between cathode terminal K1 and anode terminal A1 of the rectifier circuit to a predetermined value. This feature of limiting the potential difference is a clamping voltage and, in special cases, forms a protection against overvoltages or forms a load dump protection.
In this case, the electric strength of MOSFET transistor T1 is designed in such a way that its drain-source breakdown voltage is substantially higher than the clamping voltage of the circuit determined by clamping diode D4. For example, at a clamping voltage of 22 V, a power MOSFET transistor having a drain-source breakdown voltage of approximately 40 V is utilized. During the clamping with the aid of diode D4, transistor T1 is simultaneously operated at high voltages and currents. This is referred to as operation in current saturation or also as the linear mode.
The linear mode is unstable, in particular, in modern MOSFET transistors with their small cell structures. At high drain-source voltages and high drain currents, the current tends to constrict at a point, which may result in the destruction of the component.